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    <title>Document</title>
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    <script>

        // ***最好手动给一个默认值m=0，n=0 让函数更严谨
        function abc(n, m) { //形参 看作是一个变量，没给值 默认undefined
            document.write
            alert(n * n / m * m / n)
        }
        abc(9, 10)//实参  本质是在赋值
    </script>
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